1. Field of the Invention
Method for increasing (stimulating) the production rate of wells. More particularly, method is provided for re-stimulating the production rate of wells that have been previously hydraulically fractured.
2. Description of Related Art
In recent years new methods have revolutionized recovery of oil and gas from reservoirs. Prior methods could not recover hydrocarbons at an economic rate from the low-permeability rocks being drilled. The combination of drilling wells in a horizontal direction in shales or other low-permeability rocks and then hydraulically fracturing at intervals along the horizontal wellbore opened a whole new resource base in the industry. Typical wells are drilled from about 2000 to about 10000 ft. in the horizontal direction, but may extend much farther, and, in one common method of completing the well, perforations through casing are formed in clusters and the clusters are placed 200 to about 400 feet apart along the horizontal segment of the wellbore. Clusters of perforations may be 10-30 feet apart and the number of perforations may vary widely. A well may have more than 1000 perforations. In this “perf and plug” method of completion, a first cluster of perforations is formed near the “toe” (distal end) of the casing, a hydraulic fracturing treatment is performed through the cluster or perforations, a mechanical plug is set above that cluster of perforations, another cluster of perforations is formed above the plug and then another fracturing treatment is performed through the new set of perforations. This process may be repeated in a single well numerous times—up to 30 or 40 times or more is not highly unusual in some reservoirs. When all fracturing has been completed and plugs are removed from the wellbore, high production rates from the well may be realized.
Not only has there has been a massive amount of stimulation of long productive intervals in reservoirs in horizontal wells, but there has been continuing drilling and fracturing of thick formations in vertical wells. These wells combined have, in fact, turned the U.S. oil industry around, stemming the decline of daily oil and gas produced.
Almost by definition fracture-treated wells using proppant will, at some point, require re-stimulation. There are many reasons for this necessity. They include movement of fines in the fracture, plugging off the conductive flow path. Other reasons include deterioration of proppant, which may include crushing or simply dissolution of the proppant through produced water. Embedment with time into the fracture face, as well as scale deposition in the conductive pathways, also requires reopening fractures and reestablishing the tremendous surface area required to be able to produce hydrocarbons at economic rates from low-permeability reservoirs. Re-stimulation has been shown to be very effective in some proppant-treated wells, achieving significant amounts of production that otherwise would be lost. But the process is limited by the fact that most of these wells are left after the stimulation with hundreds if not thousands of perforations, which, using conventional techniques, prevents successful re-stimulation of these wells because the re-stimulation fluid cannot be directed into selected intervals short enough to achieve fracturing rates in the interval. Although proppant fracturing is described in detail herein, it should be understood that the same procedures described here may be applied in wells that have been acid fractured.
Commonly used techniques to control fluid flow through perforations in casing into selected intervals are ball sealers and particulate diverting materials (fiber and granules). These techniques are helpful when the number of perforations is limited to the range of dozens, but when the number of perforations is large, as in most re-stimulation treatments, ball sealers fail due to low velocity (the balls do not seat). Particulate diversion techniques do not appear to produce seals that are competent enough to allow for successful diversion of fractures into multiple intervals.
Much effort has been put forward to try to re-fracture these long perforated laterals utilizing many different types of particulate- and fiber-based diverting agents. These types of products have been a part of the industry for more than fifty years and field results do not indicate optimal stimulation. The primary problem is believed to be the lack of control of the material once through fractured perforations. Although they may temporarily divert the fluid, they typically break down and any diversion is lost. Additionally the approach used by operators is to attempt to treat a part of the interval with all of the interval open and then divert away from that stage to another. Field results have indicated minimal re-stimulation results, with most of the treatment being utilized at the heel of the lateral.
Ball sealers made of solid material (polymer) that degrades under downhole temperatures and in the presence of water have long been known (U.S. Pat. No. 4,716,964). Commercial ball sealers made of degradable materials are available in industry from Perf Sealers of Houston, Tex., or Fairmount Santrol. Degradable polymers such as polylactic acid and polyglycolic acid degrade (de-polymerize) in water at a rate dependent on temperature. Polymers can be selected to degrade over a range of temperature and time, as illustrated by ball sealers sold by Perf Sealers and designated as “LT” (low temperature), “MT” (medium temperature) and “HT” (high temperature. Degradable ball sealers are available with a range of densities, including densities approaching the density of a treatment fluid to be pumped into a well, according to Perf Sealers' web site It has been shown that matching the density of ball sealers to fluid density and injecting a mixture of balls having a range of densities can greatly increase the seating efficiency of ball sealers on perforations in a horizontal wellbore. (http://www.perfsealers.com/multi-density-perf-sealers, searched Oct. 1, 2015, citing “A New Way of Staged Fracturing Using Ball Sealers,” SPE 140529, May 2012, and “Experimental and Field Data Analysis of Ball Sealer Diversion,” SPE 147632, March 2013.)
In recent years methods of real-time monitoring the hydraulic fracturing process in a well have been developed and commercialized. Such services are available from Pinnacle, Microseismic Inc, and Weatherford, for example. Microseismic methods allow the location of the hydraulic fracture being formed to be mapped in real time at the surface of the earth. Detectors for the acoustic waves from the fracturing process may be on the surface of the earth or downhole in a well near the well being fractured or in the well being fractured.
What is needed is a method of using ball sealers having selected degradation times and densities to allow re-fracturing of multiple segments of a well that has previously been fractured in multiple segments. In one embodiment, a monitoring method to detect the location of fractures from a well may be used during the fracturing process involving injection of degradable ball sealers.